1. Field of the Invention
The present invention relates to an EGR-gas temperature estimation apparatus, for an internal combustion engine, which estimates the temperature of EGR gas flowing through an exhaust circulation pipe of the internal combustion engine.
2. Description of the Related Art
Conventionally, there has been widely known an EGR apparatus which circulates a portion of exhaust gas of an internal combustion engine to an intake passage via an exhaust circulation pipe, in order to reduce the amount of nitrogen oxides (NOx) discharged from the engine. Such an EGR apparatus is applied to both spark-ignition engines and diesel engines.
Meanwhile, in a diesel engine, combustion is effected at a super-lean air-fuel ratio. Specifically, since oxygen required for combustion is sufficiently present, the output of the diesel engine greatly depends on fuel quantity. Therefore, in a diesel engine, when the quantity of exhaust gas that is circulated so as to greatly reduce NOx emissions (i.e., the mass flow rate of EGR gas; hereinafter, simply referred to as “EGR gas flow rate”) is increased, and a sufficient quantity of fuel is supplied to the engine so as to secure a required output of the engine, the increased EGR gas flow rate decreases the quantity of new air (quantity of oxygen) which the engine takes in, whereby the ratio of the new air quantity to the fuel quantity (i.e., air-fuel ratio) shifts to the rich side. As a result, emission of particulate matter (hereinafter referred to as “PM”) increases.
In view of the forgoing drawback, there has been developed an EGR apparatus in which an EGR-gas cooling apparatus (EGR cooler) for cooling EGR gas is interposed in an exhaust circulation pipe in order to reduce the temperature of the EGR gas, to thereby increase the density of the EGR gas. The EGR apparatus can increase quantity of EGR gas, without reducing the quantity of new air, to thereby reduce emissions of NOx and PM simultaneously.
Meanwhile, for example, in the case where the above-described EGR gas flow rate is controlled on the basis of an EGR ratio, which is the ratio of the EGR gas flow rate to the flow rate of all gases taken in by an engine (hereinafter, also referred to as “intake air”), the EGR ratio must be accurately estimated, and in order to accurately estimate the EGR ratio, the quantity of intake air must be accurately estimated. Since the quantity of intake air changes depending on the temperature (intake-air temperature) as measured at a junction portion between an intake manifold and cylinders (at the outlet of the intake manifold), the intake-air temperature must be accurately estimated. Further, in order to accurately estimate the intake-air temperature, the temperature of new air and the temperature of the EGR gas immediately before being mixed with the new air (the latter temperature is substantially equal to the EGR-gas temperature as measured at the outlet of the EGR-gas cooling apparatus) must be accurately estimated. In other words, accurate determination or estimation of the temperature of the EGR gas after being cooled by the EGR-gas cooling apparatus is extremely important for appropriate control of the engine.
In view of the above, a conventional EGR apparatus equipped with the above-described EGR-gas cooling apparatus obtains a value k corresponding to the efficiency (cooling efficiency) of the EGR-gas cooling apparatus on the basis of engine speed and fuel injection quantity, and obtains its correction coefficient kh on the basis of EGR gas flow rate. Subsequently, the conventional apparatus estimates the EGR gas temperature Tegr at the outlet of the EGR-gas cooling apparatus by use of the above-described value k, the above-described correction coefficient kh, area A of a heat transfer surface of the EGR-gas cooling apparatus, flow rate G of EGR gas, specific heat Cp of EGR gas, temperature Twg of EGR gas cooling water, exhaust temperature Tex, and an expression Tegr=Tex−Twg·k·A/(k·kh·A/2−G·Cp) (see, for example, Japanese Patent Application Laid-Open (kokai) No. 11-166452 (page 6, FIG. 11, and FIG. 14)).
However, even when the flow rate of EGR gas flowing into the EGR-gas cooling apparatus is constant, the cooling efficiency (heat conductivity, or heat transfer ratio) ηegr of the EGR-gas cooling apparatus changes greatly with the temperature of the EGR gas flowing into the EGR-gas cooling apparatus (this temperature is substantially equal to the EGR temperature as measured at an inlet of an exhaust circulation pipe where the exhaust circulation pipe is connected to the exhaust passage). Specifically, as shown in FIG. 17, even when the flow rate of EGR gas flowing into the EGR-gas cooling apparatus is constant, the cooling efficiency ηegr changes from the value at point A to the value at point B when the temperature of the EGR gas flowing into the EGR-gas cooling apparatus changes from a first temperature to a second temperature higher than the first temperature. Such a phenomenon occurs, because the greater the difference between the temperature of the EGR gas flowing into the EGR-gas cooling apparatus and the temperature of coolant of the EGR-gas cooling apparatus, the greater the quantity of heat that is taken from the EGR gas by the EGR-gas cooling apparatus.
Accordingly, the above-described conventional technique that estimates the cooling efficiency of the EGR-gas cooling apparatus without consideration of the temperature of the EGR gas flowing into the EGR-gas cooling apparatus has a problem in that the temperature of the EGR gas at the outlet of the EGR-gas cooling apparatus cannot be accurately estimated.